I have some money to spend on a new postprocessing workstation; I'm looking at a dual Xeon machine with 48GB RAM for looking at really big datasets. I have a choice of graphics cards, and I'm uncertain which to go for. I usually just choose the most expensive one I can afford, which in this case is the nVidia Quadro FX3700 with 512MB GDDR3. However I happened to notice that the machine can also be supplied with an nVidia GTX295 which provides 1792MB GDDR3; at around 1/2 the cost of the Quadro card. I understand that the Quadro cards are intended as "professional" cards; whilst the GTX series are "gaming" cards; the sysadmin here in Exeter recommends going for the Quadro as being more robust as a result. However I wonder, given the large datasets I am intending to process on this machine, whether the GTX card would be better, simply as a consequence of the large memory associated with its GPU. Does anyone have any guidance/recommendations/experience with any of this that they would be able to share with me, please?

Go with the GTX295 or run 3 GTX275's.
The GTX275 is openGL 3.0.
I'm using it and it's very solid.
The story behind the "Professional cards" is that the Game cards and the Professional cards are identical in the hardware, the difference is in the drivers.
The game cards are mostly optimized for DirectX. However the lastest generation models are OpenGL.
The reason for the extra cost.
OpenGL is a standard that contains common functions used across many CAD programs, however some programs have different functions in their graphics pipeline that are not in the Open standard hence the drivers need to be customized for these programs. Hence the cost.
If you are running a particular CAD program that requires the Quadro Card then yes you would need that card other wise it's a waste of money. Plus the game cards are oc and if they burn out just get a new one or three.

I definitely agree. For roughly 10 million cells on a high-resolution monitor, you'll notice a few fps difference between both cards anyway. I'd invest that saved money in either more RAM or go for dual quads (which you may have already). Enjoy the system when you get it! :-)